In recent years, attention is focused on various flexible electronic devices which are required to be lightweight, thin, or flexible in shape. Plastic film is currently used instead of glass, which was used conventionally, as base material of flexible electronic devices, but such plastic film used as base material of electronic devices has serious disadvantages such as deterioration in thermal dimensional stability and curling properties that can result from thermal expansion and thermal shrinkage during steps for forming transparent electrically conductive layers or luminescent/power generating layers.
Having good heat characteristics, dimensional stability, mechanical characteristics, electric characteristics, heat resistance, and surface characteristics, biaxially oriented polyethylene terephthalate film has been in wide use as base material for various products including magnetic recording material, packaging material, electric insulation material, various photographic materials, graphic art material, and optical display material. It is thought, however, that base films for flexible devices with improved physical properties have to be developed, and efforts made in the past to provide polyethylene terephthalate film with high characteristics have disclosed a method that blends thermoplastic resin with polyethylene terephthalate (Patent document 1), a method that adds a large quantity of particles to improve the low thermal expansion property (Patent document 2), a method that blends thermoplastic resin with polyethylene terephthalate resin to form a sea-island structure, thereby improving thermal dimensional stability (Patent document 3), and a method that adds an inorganic filler to polyester resin to improve thermal dimensional stability (Patent document 4).